In the next generation communication system, also known as the 4th Generation (4G) mobile communication system, a cell has a significantly smaller radius in order to provide high-speed communication and to handle a large amount of call traffic. Currently, a wireless network is designed based on a centralized design. However, it is expected that the centralized design cannot apply to the 4G communication system. Therefore, the 4G communication system has to be controlled in a distribution manner and also has to be able to actively cope with changes in environments, for example, when a new base station (BS) is added. For the aforementioned reason, a self-configuration wireless network is required in the 4G communication system.
In order to realize the self-configuration wireless network required in the 4G communication system, a technique applied in an ad-hoc network is introduced in the wireless communication system. A representative example thereof is a multi-hop relay cellular network in which an ad-hoc multi-hop relay scheme is introduced in a cellular network which is constructed of a fixed BS.
FIG. 1 illustrates a structure of a conventional cellular network using a multi-hop relay scheme.
Referring to FIG. 1, a mobile station (MS) 110 included in a BS coverage area 101 is connected to a BS 100 with a direct link, and an MS 120 located out of the BS coverage area 101 and therefore having a poor channel state with the BS 100 is connected to the BS 100 via a relay station (RS) 130. In this case, since the RS 130 serves to relay a signal between the BS 100 and the MS 120, a BS-MS link, a BS-RS link, and an RS-MS link are formed. For example, when an MS is located in a cell edge of the BS 100 or in a shadow area suffering a serious shielding phenomenon due to buildings, the MS communicates with the BS 100 via the RS 130. As such, a high-speed data channel can be provided by using a multi-hop relay scheme in a cell edge region having a poor channel state, and it is also possible to expand a cell service area.
The broadband wireless access system generally provides an Internet service, a Voice over Internet Protocol (VoIP) service, a non-real time streaming service, etc. In addition, recently, a Multicast and Broadcast Service (MBS), which is a real-time broadcast service, was recognized as a new service. Advantageously, the MBS can provide bidirectional data communication and also support the same mobility as a Digital Multimedia Broadcasting (DMB) service. The MBS can provide a video broadcast service (e.g., news, soap opera, sports, etc.) and a data service (e.g., radio music broadcast, real time traffic information, etc.).
According to a service access method of an MS, the MBS is classified into two categories, that is, a single-BS access method and a multi-BS access method. In the single-BS access method, the MS receives an MBS service from one BS to which the MS is registered. In the multi-BS access method, the MB receives the MBS service simultaneously from two or more base stations.
FIG. 2 illustrates a multi-BS access method.
In the multi-BS access method, when an MS is located in an overlay area between a currently serving cell and a neighboring cell, a signal of the neighboring cell dose not act as noise caused by interference but acts as a signal gain resulted from radio frequency (RF) combining. This is a macro diversity effect. In order to obtain the macro diversity effect, the same signal is transmitted from a serving BS and a BS located in the neighboring cell. Therefore, to provide the MBS, all base stations and all relay stations exiting within a broadcast zone (i.e., MBS_ZONE) transmit the same signal.
When a BS has data (e.g., MBS data) to be transmitted at a specific time point, the BS considers a processing delay which occurs in an RS, and thus transmits the data to the RS before the specific time point. By doing so, even when the RS experiences the processing delay, for example, to analyze the received data or to allocate a bandwidth for retransmission, the BS and the RS can transmit the same data at the same time point.
For example, when the data is transmitted through two-hops, the BS transmits the data in advance by considering not only a first-hop RS but also a second-hop RS. In this case, in consideration of the processing delay occurring in the second-hop RS, the first-hop RS directly connected to the BS buffers the data, and transmits the data to the MS at a predetermined time point.
Meanwhile, the broadband wireless communication system may cause interference in UpLink (UL) communication when neighboring base stations use the same frame number at the same time point. For this reason, in order to increase a system capacity or coverage, the neighboring base stations need to use different frame numbers from one another. The same also apply to a system using a relay scheme. Hence, it is preferable that base stations and relay stations use different frame numbers at the same time point. In this case, if a frame number cannot be differentiated between a BS and an RS, a problem may occur in which a message to be transmitted at the same time point or at the same frame number is transmitted by the BS and the RS at a different time point or at a different frame number. Therefore, when the different frame number is used between the BS and the RS, there is a need for a signaling process capable of negotiating a frame offset between the BS and the RS.